I have one of these on my '50 Chevy COE and another on my '47 Diamond T, model 509. The latter is like yours and is vacuum actuated. The Chevy is actuated mechanically by the 2 speed selector switch. It is the same unit minus the diaphragm (and can be converted by removing it). The diaphragm tends to crack and then will fail to function due to vacuum leakage, so if yours is intact it is a valuable piece.

Kent

1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix